In the field of catalysis, such as in the field of catalysts for the treatment of automotive exhaust, catalysts are employed wherein catalytically active materials are usually applied to a monolithic support member. In most of the respective applications, flow-through monoliths are used which comprise channels with walls separating the channels from each other. In most cases, such channels have polygonal cross-section wherein rectangular, in particular square cross-sections are preferred. If the catalytically active materials are applied onto the walls of these channels having rectangular cross-section, it is a common problem that in the corners formed by two adjacent walls, a fillet portion of the coating is formed during application of the coating. These fillet portions roughly define a concave profile in cross-section, and according to known processes of the art such as, for example, dip coating processes, the radius of this concave profile is comparatively large. This in turn means that a comparatively high amount of coating and, thus, a comparatively high amount of catalytically active material which is contained in the fillet portions is not accessible for the exhaust gas stream and, therefore, useless for the desired application of the coated support member as catalyst. Due to the fact that for several catalytic purposes, such catalytically active material comprises at least one precious metal component, such waste of catalytically active material in the fillet portions is also a considerable economic disadvantage. Further, the larger said concave radius of the fillet portions, the lower is the surface which is accessible for the exhaust stream within a given channel of the monolithic support member. Therefore, large radii will lead to an undesired loss of catalytic activity of the coated monolith. Also, an undesired increase of pressure loss can be expected from large radii.
DE 199 61 483 B4 discloses catalysts for the purification of the exhaust gas of an ignition engine. According to this document, monolithic support members are disclosed with hexagonal cells wherein the thickness of a catalytically active layer coated onto the walls of the monolith is 10 to 70 micrometer in the thinnest areas and wherein, in the thickest areas, the thickness is not more than 12 times the thickness of the thickness in the thinnest area. In particular according to FIG. 3 of DE 199 61 483 B4, avoiding large radii of the fillet portions is not achieved.
U.S. Pat. No. 4,335,023 discloses monolithic support members supporting catalytic promoting materials wherein the channels of said members are nominally polygonal, preferably square in cross-section profile. In the context of U.S. Pat. No. 4,335,023 it is described that the juncture of the channel walls are filleted to provide a concave profile. Thus, by changing the monolithic support member as such, preventing or at least reducing the accumulation of excess catalytic material which occurs in the sharp angular corners is achieved. However, changing the geometry of the monolithic support member as such will lead to an increased difficulty in preparing the monolith. According to FIG. 5 of U.S. Pat. No. 4,335,023, stippled portions AB are shown in the juncture corners of the walls which is not readily accessible. FIGS. 2, 3 and 4 of U.S. Pat. No. 4,335,023 show coated monolithic support members which are described in detail in table A in column 9. According to this table, the smallest radius of curvature of the coating in the fillet portions is 0.0168 inches, corresponding to 0.043 cm. As discussed above, for avoiding too large a radius, U.S. Pat. No. 4,335,023 teaches to change the monolithic support member, and no teaching is given as far as an improved method for coating common monolithic support members is concerned.
Therefore, it was an object of the present invention to provide a method for coating a monolithic support member comprising channels with walls separating the channels from each other, the channels having a polygonal cross-section profile, wherein in the coated monolithic support member, the junctures of two adjacent coated walls are formed by coating fillet portions, and wherein a coating fillet portion defines in cross-section a concave profile of a depth extending to the midpoint of a segment of a circle taken as disposed tangentially with respect to the two adjacent coated walls, wherein said circle has a small radius R.
Surprisingly, it was found that it is possible to manufacture such coated monolithic support members by a method wherein a suspension exhibiting a specific solid content is dispersed in a gas stream and the gas stream is directed towards the monolithic support member along the axial direction of the channels of the support, wherein the mass flow of the dispersed suspension as well as time for which the dispersed suspension is directed towards the monolithic support member are in specific ranges.